On Thu, Apr 3, 2025 at 12:11 AM Sanjay Govindjee via petsc-users < petsc-users@mcs.anl.gov> wrote:
> We would like to solve an FEA problem (unstructured grid) where the nodes > on the elements have different dofs. For example the corner nodes have > only dof 0 and then mid-side nodes have dofs 0,1,2 (think 8 node > serendipity element). This is a multi-physics problem so we are looking to > use the fieldsplit features to pre-condition and solve. Is there a simple > example of this type of usage in the src that we can try to mimic? > > I presume this will take programming as opposed to just setting command > line options. > It will take a little programming, but not much. Here is the idea. FieldSplit needs to know what dofs belong to what field. There are a couple of ways to do this, at different levels of abstraction. 1. Low level You can explicitly makes lists of the dofs in each field, as an IS, and call https://urldefense.us/v3/__https://petsc.org/main/manualpages/PC/PCFieldSplitSetIS/__;!!G_uCfscf7eWS!aMROjbrPD3RYXMpO8mIii7q8eXZX1uN-6F6-g_jcNLLXGCgYPt2JEDkIIQCHs_vNBhhxsiwQJaz57ydxKyWe$ for each field. This is not very flexible, but the easiest to understand. 2. Intermediate level You can make a DMShell, and then make a PetscSection, that gives the number of dofs on each vertex and edge. Then call KSPSetDM() or SNESSetDM(), and you can do nested fieldsplits from the command line. This also retains a connection between the topology and the data layout, but you have to deal with that pesky DM object. 3. High level You can use a DMPlex to represent your grid and a PetscFE to represent the discretization, and then layout is done automatically, and nested fieldsplits can be done from the command line. I am not 100% sure PetscFE can represent what you want, but you can always call DMPlexCreateSection() by hand to make the PetscSection. Thanks, Matt > -sanjay > > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener https://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!aMROjbrPD3RYXMpO8mIii7q8eXZX1uN-6F6-g_jcNLLXGCgYPt2JEDkIIQCHs_vNBhhxsiwQJaz579za_RM4$ <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!aMROjbrPD3RYXMpO8mIii7q8eXZX1uN-6F6-g_jcNLLXGCgYPt2JEDkIIQCHs_vNBhhxsiwQJaz57-FUThx2$ >
